A search for "Celtic Christianity" will turn up all sorts of sites ranging from neopagan to evangelical Baptist. However, I have found one site to be quite authentic and very helpful. The Celtic Orthodox Christian Church is based primarily on the Stowe Missal (AD 750 Irish manuscript translated from Gaelic) and the Antiphonary of Bangor (completed circa AD 680-691). Of course, both of these manuscripts were based on much earlier traditions.

You can download an English translation of the Stowe Missal here: http://celticchristianity.org/library/stowe.pdf
This also includes many notes on the prayers, the mass, etc. A wealth of information for understanding how they thought, why they did what they did, etc.

"Bless the Lord at all Times" is their altar missal, which includes the mass, lives of the saints, funeral service, etc.: http://celticchristianity.org/library/blessthelord.pdf
How would a Celtic monk pray for a dying knight or minister unction? You'll find that here. 800 pages!

Here is the rule of life for Celi De (aka Culdees, Celtic monks, lit. "vassals of God"): http://celticchristianity.org/library/regula.html

Here is a perpetual calendar for the Celtic Christian year: http://celticchristianity.org/library/perpetualcalendar.pdf
For instance, today 6 Sept was St Macc Cuilinn, St Sciath and St Colomb of Ross Giallain.

If you need to find Easter for your game year, here is another useful site: http://www.smart.net/~mmontes/ec-cal.html
Easter in AD 485 was April 21, by the way.

You might also find the Carmina Gadelica useful, though these are more 19th century Scottish folklore, but some prayers or charms could be useful: http://www.sacred-texts.com/neu/celt/cg.htm

Here is info on the Celtic vestments: http://celticchristianity.org/library/vestments.html

Hope this helps flesh out some characters and setting for you.